Welcome to Memory Mondays, where I read a textbook on memory and talk about what I Iearned. If you like your cognitive psychology neatly summarized, with a healthy dose of unnecessary commentary and bad humour, this is the series for you!
While the text focuses on a psychological approach, evidence from neuroscience is still woven in, as it was in the cognitive psychology textbook I read previously. Alas, it seems I cannot fully avoid learning about the parts of the brain.
For example, if evidence from psychological studies shows that verbal and visuo-spatial short-term memory (STM) are separate, then neuroscience evidence that shows visual and spatial activities activate different parts of the brain supports the theory that they are indeed separate systems.
The case of Clive Bearing in the last Memory Mondays post is an example of the neuropsychological approach. Patients with brain damage are studied for relationships between the location of the damage and the nature of the resulting problems. This approach permits not only localization of cognitive functions (figuring out where in the brain they are), but also dissociation (seeing whether cognitive capacities are separate). Researchers seek double dissociations, where two patient groups show opposite deficits: for example, one group has normal verbal STM and impaired visuo-spatial, and the other impaired verbal STM and normal visuo-spatial.
While famous cases such as Bearing are important, they are rare, especially those where just one cognitive capacity is impaired. Other evidence is needed to further develop models, including that from healthy subjects.
Observing the Working Brain
The book touches on how researchers look at the structure of the brain, but we’re just interested in the brain in action, so we’re going to skip that part.
The first method discussed for looking at the working brain is recordings of single neurons, where electrodes are placed in the brain to record electrical activity. Due to ethics, this is only done on patients who are undergoing surgery for other reasons, like for the treatment of epilepsy. So, like with neuropsychology, these are not healthy patients.
There are also non-invasive methods for measuring electrical and magnetic activity.* Electroencephalography (EEG) measures the former, using electrodes on the scalp. I once participated in a study using this method, while I was in university and part of the pool of readily available psychology subjects. The study was trying to measure event-related potentials, electrical activity in response to a given event, by repeatedly presenting the stimulus and averaging together the responses; the repeated part is important, as it takes a lot of data points to cancel out the noise. In layman’s terms, I had to do the same thing over and over again, with a net soaked in salt water over my scalp. The computer ended up crashing, so we didn’t finish the experiment, but I still got full pay. Overall, it was an interesting experience.
Magnetic activity can be measured with magnetoencephalography (MEG), which is less distorted by the scalp and thus has better spatial resolution.
The disadvantage of these methods is that they can show that these areas of the brain are activated during a task, but not whether that area is necessary for the task. Transcranial magnetic stimulation (TMS) does. Electric current passes through a coil placed close to the participant’s head, which creates a temporary lesion. If performance is impaired, then researchers can be confident that part of the brain is necessary for that cognitive function. TMS, however, is limited to the surface of the brain.
These methods based on electric or magnetic activity, with the exception of single neuron recordings, have poor spatial resolution; it is difficult to be precise with where activity is happening. Blood flow based measures are better in that regard. They operate on the assumption that when an area of the brain is active, it requires more oxygen, and thus blood flow to the area increases. Positron emission topography (PET) measures this flow using radioactive tracers in the blood, while functional magnetic resonance imaging (fMRI) measures the change in magnetic signal of the blood as oxygen is depleted. The main disadvantages of fMRI are the cost and the necessity of carefully designing the tasks, while for PET the problems are the costs and, well, putting radioactive substances in people’s blood.
More recently, computers have assisted in the interpretation of fMRI scans. The visual representation from the scan is divided into tiny areas called voxels; a computer will look for patterns that occur when the same event is repeated. The results are quite cool; when new scans are presented, the computer can “mind read” what stimulus was presented to the subject.
While many different approaches can be used to study the brain, what is important is convergence. Even if a single study can be subjected to many interpretations, only one explanation will be able to account for a range of studies. While having different approaches for studying the brain is more for me to wrap my mind around, in the end, the theories will be more robust when supported by a variety of evidence.
Next week: short-term memory
*We measure electrical activity because neurons use electrical signals to communicate; we measure magnetic activity because this electrical activity creates magnetic fields.